Ultra-high strength thermo-mechanically processed steel

ABSTRACT

The present invention disclosed an ultra-high strength steel for structural components, a process of making such steel that has a desirable microstructure in the thermo-mechanically processed and differently cooled conditions that delivers high fatigue performance in service, and a process of making forged components using such steel. The steel and the process of its manufacturing enables manufacture of components that exhibit bainitic microstructure that impart ultra-high strength ranges with very high fatigue performance. The invention enables saving in alloying additives compared to hardened and tempered alloy steels and in addition avoid expensive heat treatment operations to achieve the desired range of mechanical properties. The steel of the invention is a suitable replacement for micro alloyed steel or heat treated steel bars used for structural component development. The steel can be used for applied as the hot rolled and air cooled long products that can be directly used for applications or it can be directly hot forged in open or closed die forging followed by controlled cooling to achieve the desired microstructure and range of mechanical properties.

FIELD OF THE INVENTION

The present invention relates to ultra-high strength steel for structural components. In particular, it relates to the process of making such steel that has a desirable microstructure in the thermo-mechanically processed and differently cooled conditions that delivers high fatigue resistance in service. The steel and the process of its manufacturing technique enables manufacture of components that exhibit bainitic microstructure that impart ultra-high strength ranges with very high fatigue resistance properties.

The steel and its method of manufacturing process enables saving in alloying additives compared to hardened and tempered alloy steels and in addition avoid expensive heat treatment operations to achieve the desired range of mechanical properties. The steel developed in the present invention is a suitable replacement for microalloyed steel bars used for structural component development. The steel can be used for applied as the hot rolled and air cooled long products that can be directly used for applications or it can be directly hot forged in open or closed die forging followed by controlled cooling to achieve the desired microstructure and range of mechanical properties. The ultra-high strength steel (UTS in the 1100 to 1420 MPa range) of the invention provides a competitive replacement for quench tempered alloy steels in terms of strength and fatigue, ductility, although its toughness is lower compared to Quenched and Tempered steels but superior to micro alloyed steels. Thus, the potential steel bar product can be applied in various industries that include automotive, railways, general engineering, agricultural implements, construction, mining shafts, etc. The steel exhibits unique range of mechanical properties that develops due to the process of invention which involves the steel chemistry and the thermo-mechanical processing followed by controlled cooling.

BACKGROUND OF THE INVENTION

High performance and weight reduction of dynamic components in engineering machines is a major driver for achieving fuel efficiency. In automotive industry, even small weight reduction in dynamic engine components translates into significant fuel efficiency and overall weight savings in the structure. Products such as bars, tubes/pipes, and other solid products such as axles, typically termed as ‘long’ products, made of steel and produced using minor addition of additives such as Vanadium (V), Niobium (Nb) and Titanium (Ti) and with ferrite pearlite microstructures have long dominated the markets. These materials, which have been competing with alloy steel in terms of achieving the required strength, are produced using thermo-mechanical processes with controlled cooling, which completely avoid subsequent quenching and tempering heat treatment of the component. The micro alloyed steels are known to have ultimate tensile strength (UTS) upto 1000 MPa although they have lower impact toughness.

The present invention relates to innovative composition of a steel and associated thermo-mechanical processing of making it, such that it has desirable microstructure that gives ultra-high strength and fatigue resistance. The mechanical properties of the invented steel shows values of ultimate tensile strength (UTS) between 1100 and 1420 MPa; yield stress (YS) between 700 and 1030 MPa; percentage elongation (% E)=10 to 18%; % reduction in area between 32 and 50%, Charpy V-notch Impact toughness greater than 20 J/cm² and hardness greater than 330HBW. Higher strengths were obtained with increasing cooling rate after thermo-mechanical processing. The fatigue properties of the steel rolled bar of the invention have been found to exhibit greater than 5 million cycles with rotating bending type testing at a stress level of 515 MPa. It was also found that the fatigue life of the forged component of the invention is at least five times more than that of the existing alloy steel materials.

SUMMARY OF THE INVENTION

The present invention describes the steel composition and the entire process of making the steel from melting to thermo-mechanical processing to achieve steel with ultra-high strength and very high fatigue performance. The ultra-high strength steel of the invention has Carbon in the range of 0.1 to 0.25% which provides the desired microstructure and the type of nano-carbides. It has Manganese (Mn) content between 1.2 and 2.5% and Chromium (Cr) content between 0.8 and 1.4% which helps the steel achieves the desired bainitic bay in the CCT diagram and the elements increase the hardenability. It also has Silicon (Si) level of 0.5 to 1.7% to achieve the desired microstructure in control cooled conditions. To improve the prior austenite grain structure, the steel is alloyed with 0.05 to 0.1% Niobium (Nb) content. The effect of dynamic strain aging associated with Nitrogen is prevented by addition of 0.01 to 0.03% Titanium (Ti). Molybdenum (Mo) to a level between 0.05 to 0.1% is added to the steel to promote and stabilise the bainitic microstructure. Boron to a level of 30 ppm was added to ensure hardenability over thicker cross sections. The steel is aluminium killed with a residual Aluminium (Al) level <0.02%. The steel can have residual elements Nickel (Ni) in an amount up to 0.4% and Vanadium (V) up to 0.1%, sulphur (S) content <0.03% and phosphorus (P) content <0.02%.

The steel of the invention can be manufactured through electric induction or electric arc furnace or basic oxygen furnace route followed by alloying in a ladle or induction furnace. The steel may be processed by ingot casting or continuous casting route, while the latter is preferred to achieve for higher yield and productivity. The cast ingot or continuous cast blooms are hot worked at a temperature range of 1250° to 900° C. in a forge press or in a hot rolling mill or hot extrusion or other related to semi or finished products. The finishing temperatures are maintained in the range up to 1000 to 900° C. Then as-hot worked steel is allowed to cool in air, or quenched in oil, water or polymer. The water quenched sample may have up to 7% martensite depending on section thickness. In the manufacturing processes, other than air cooling, a stress relief tempering heat treatment at about 200 to 340° C. is optionally employed.

Objects of the Invention

The object of the present Invention is to develop an ultra-high strength steel with high fatigue resistance with thermo-mechanically processed and control cooled steel that is processed with induction or electric arc furnace or Basic oxygen furnace routes followed by secondary refining, vacuum degassing and traditional casting processes such as ingot casting or continuous casting.

Another object of the present investigation is to achieve a wide range of mechanical properties within Ultra high strength range by changing the cooling rate post thermo-mechanical processing in air, water, polymer, oil and vermiculite cooling.

Another object of the present invention is to propose usefulness of steel in weight reduction by atleast 10% as a substitute for heat treated alloy steels or microalloyed steels.

Another object of the invention is to minimize the cost of steel manufacturing as compared to alloy steels and savings associated with avoidance of heat treatment in alloy steels.

Another object of present invention is to produce ultra-high strength components having fatigue resistance of at least five times compared to regular heat treated or micro alloyed steels.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 Manufacturing method process flow of ultra-high strength steel.

FIG. 2 Microstructure of the steel made at various cooling conditions showing the bainitic microstructure obtained at various processing conditions

FIG. 3: Automotive front axle beam manufactured using ultra-high strength steel grade.

FIG. 4 Microstructure of the forging with invented ultra-high strength steel.

DETAILED DESCRIPTION OF THE INVENTION

The present invention describes the development of ultra-high strength steel for potential use in light weighting and fatigue resistant component applications.

The primary steel is manufactured using induction furnaces, electric arc furnaces (EAF), ladle furnaces or basic oxygen furnaces. The primary steel is suitably alloyed in a secondary refining furnace followed by vacuum degassing to produce the steel of designed chemistry additives shown in Table 1, based on theoretical considerations on alloy design principles. The steel is refined and killed with Aluminium (Al) to get a residual oxygen level to less than 15 ppm. This is found to reduce the potential oxide inclusions that have an effect on the fatigue life of the component that may be made using the steel of the present invention.

TABLE 1 Composition of the steel designed for meeting Ultra high strength range and fatigue resistance. [Composition in % w/w] Elements C Mn Si S P Cr Ni Mo V Al B Ti Nb O Min 0.10 1.2 0.5 — — 0.8 0 0.05 — — — 0.010 0.05 Max 0.25 2.5 1.7 0.03 0.02 1.4 0.4 0.1 0.1 0.02 0.003 0.035 0.10 0.0015

The steel is cast through ingot casting preferably with bottom pouring up-hill casting technique or it may be cast as a concast steel product of suitable size. The as-cast ingot or the concast bloom may be hot charged or cold charged in a furnace for further deformation. The initial soaking for hot deformation is done at a temperature between 1280° C. and 1220° C. The soaked bloom is subject to hot forging or hot rolling. Suitable reduction per pass is applied. The material gets deformed easily by forging or directly hot rolling with good surface finish. The hot worked material may be suitably hot finished at a temperature between 1000 and 900° C. This is followed by control cooling the deformed steel in any of the cooling medium vermiculite, air, oil, polymer or water. The distortion of the steel is depending on the quenching severity of the media. The steel quenched in water or polymer may require a stress relief temper treatment at a temperature between 200 and 340° C.

The invention discloses development of the steel and its processing where the content, morphology and distribution of the phases ensure the unique range of mechanical properties. The bainitic transformation involves a displacive transformation followed by diffusion of carbon to form carbides that create the bainitic microstructures.

The invention discloses the stability of predominantly bainitic microstructure when control cooled in various media such as vermiculite, air, oil, polymer and water quenching which resulted in higher strength.

The invention also discloses development of steel which can be made at par with that of processing an air cooled micro alloyed steel but delivers ultra-high strength level and superior fatigue resistance.

In one typical embodiment, the primary steel was melted in a 35MT electric arc furnace followed by secondary steel making using a ladle furnace. The entire process flow followed is shown in FIG. 1. The process of the present invention ensures that the steel developed by adopting above mentioned process has Oxygen content less than 15 ppm (a condition that is necessary to give ultra-high strength). Secondary steel making followed by vacuum degassing ensures low gas content. One advantage of completely air cooled bainitic structure is that the transformation has a natural tendency to reject hydrogen gas and the steel would be virtually free from hydrogen flaking unlike that in an alloy steel. The steel in the present case was manufactured by continuous casting while ingot casting is also possible. Macro segregation was minimised by having low carbon and sulphur contents. The steel can be hot or cold charged during deformation. The as-cast steel is amenable for hot deformation using hot forging and hot rolling and in this specific case it was directly hot rolled with a reduction ratio more than 4 with a cross section of round corner square.

The hot deformation temperature at which thermo-mechanical processing is carried out is between 1280 and 850° C. The plastic deformation of the steel happens with good plastic flow behaviour and with no surface defects. The finish rolling temperature was maintained in a range between 1000 to 900° C. The steel after thermo-mechanical processing was subject to initially three different cooling rates using vermiculite cooling, air cooling or water quenching. Each of the three cooling methods leads to the formation of predominantly bainitic microstructure. The typical mechanical properties achieved in the 35 MT directly hot rolled steel are shown in Table 3. The corresponding microstructures are shown in FIG. 2.

TABLE 2 Properties of raw material achieved at three cooling conditions. Hard- Type of UTS YS CVN ness cooling Direction (MPa) (MPa) % E % RA (J) HB Vermic- Longi- 1140 767 14.8 45 12.41 332 ulite tudinal Cooled Transverse 1108 762 10.2 25 10.45 Air Longi- 1137 761 17.8 46 17.6 340 cooled tudinal Transverse 1178 784 12 31 11.3 Water Longi- 1195 861 16 30 20.58 352 quenched tudinal Transverse 1189 850 9.8 29 9.15

This example shows that the steel is amenable for processing to bainitic transformation after thermo-mechanical processing over a wide range of temperatures. This implies that the steel exhibits bainitic structure with lesser control of cooling rate. This simplifies the manufacturing process in closed die forging where in some cases desired cooling rate is imposed to get a specific microstructure. The present invention confirms that there is no need for forced air cooling or a very controlled cooling in the conveyor after deformation. The steel made using the process of invention was used in making of close die steel component, where excellent consistency in mechanical properties is achieved.

The mechanical properties obtained in this study shows ultra-high strength range consistent in a product. Such levels of properties are usually achieved in hardened and tempered steels. In the case of the present invention, there is no need for hardening and tempering and air cooling is sufficient to achieve the desired mechanical properties. It was also observed that the transverse properties as shown in Table 2 significantly improved when the steel was subject to thermo-mechanical processing in a closed die forging operation.

The steel bar manufactured using the invented process, are easy to forge into complex shaped components. The forging made of such steel had the desired bainitic structure. This ultra-high strength steel contributes to weight reduction of the existing components that enhances fuel efficiency in automotive type applications. The component weight reduction can be achieved by at least 10%.

In an extended study, the steel made using the process of invention was used for forging an automotive front axle beam designed for Gross Axle Weight Rating (GAWR) of about 7 tons as shown in FIG. 3. The closed die forging of components made using the steel of the invention can be manufactured using a hammer or a press, and the forging process involves soaking of rolled bar at a temperature range between 1280° C. and 1220° C. The Finish forging temperature is maintained within temperature range 1000 to 900° C. The forging process involves preform manufacturing step for blocker forging and finisher forging, the process of preform manufacturing consist of reduce rolling the heated billet and bending some portion using bender tool. The finish forging is followed by cooling the hot forged component in any of the cooling medium selected from a group comprising controlled cooled, air, oil, or polymer. The microstructure of forging component using the ultra-high strength steel is as shown in FIG. 4.

It should be noted that the steps of forging process disclosed in the example above are specific to the axle beam type components. Forging process for other types of components manufactured using the steel of the invention may vary depending on component geometry.

The mechanical properties and fatigue testing of the beam was carried out. The mechanical properties and the performance of the steel are as given in Table 3. The test results shown superior fatigue properties of steel of the present invention as compared to the conventional quench tempered medium carbon steel AISI 1045 and alloy steel DIN 40Cr4.

TABLE 3 Typical mechanical property performance of the steel of the invention after forging into components. Hard- Type of YS UTS Impact ness cooling (MPa) (MPa) % E % RA (J) HB Air 700-800 1100-1200 13-18 35-45 20 J 331-352 Cooled Oil 890-980 1300-1380 10-15 32-50 23 J 412-425 Quenched Water  980-1030 1350-1420 12-17 34-50 27 J 417-438 Quenched

The fatigue test results shows at least five times better fatigue life as compared to conventional heat treated steel grades such as AISI 1045, 40Cr4. Axle beams made from the invention of the present steel can be made slimmer than those made with micro alloyed steel grades (30MnVS6+Ti) resulting in weight reduction.

Benefits of the Steel of the Invention Over Other Steel Grades:

The steel shows dense sheaves of bainitic ferrite with nano-carbides. The strengths developed in this invention far surpass the traditional medium carbon microalloyed steel grades used for component fabrication.

One key feature of the process of the present invention is that the enhanced strength of the steel disclosed herein is achieved without heat treatment by cooling in air or cooling in other media after thermo-mechanical processing. The mechanical properties of the steel of the invention are comparable to that in the oil hardened and tempered alloy steels and superior to that obtained in ferrite pearlitic medium carbon microalloyed steel.

Unlike alloy steels which obtain ultra high strength levels after quenching and low temperature tempering, the present steel achieves the strength value merely using thermo-mechanical processes and with suitable cooling medium. The steel of the present invention is comparable with 1% Cr alloy steel in terms of cost but it has mechanical properties which are equivalent to alloy steels with further alloying additive and heat treatment. The enhanced strength enables reduction in weight of the components in range of at least 10% in case of the traditional medium carbon micro alloyed steel. The improved strength values have resulted in improved fatigue life of components made from the steel of the present invention is at least 5 times when compared with those which were made using traditional micro alloyed steel grade. In the case of automotive components, the steel of the present invention is useful for light-weighting opportunities in applications such as shafts, axle beams, steering knuckles, connecting rods, camshaft, etc.

In summary, the present invention has the following aspects and advantages:

-   -   1. A robust chemistry of the steel was developed from as-cast         ingot or concast product which can be hot forged or hot rolled         followed by controlled cooling in media such as vermiculite         cooling, air cooling, oil cooling or polymer show         ferrite-carbidic bainitic microstructure that exhibits ultra         high strength levels with good ductility and reasonable         toughness.     -   2. The steel has a composition in weight percentage Carbon of         0.1 to 0.25%, Manganese 1.2 to 2.5% Silicon of 0.5 to 1.7%,         Chromium of 0.8 to 1.4%, Molybdenum of 0.05 to 0.1 wt. %,         Niobium of 0.05 to 0.10 wt. %, Titanium of 0.01 to 0.03 wt. %         and Boron between 30 ppm, with residual elements such as Nickel,         Vanadium, Sulphur and Phosphorous permissible in the following         proportions: Nickel less than 0.4%, Vanadium up to 0.1 wt. %,         Sulphur less than 0.03%, and Phosphorus less than 0.02%. Further         the Oxygen is maintained at a level of less than 15 ppm to         ensure cleanliness for the ultra-high strength range.     -   3. The steel which is thermo-mechanically processed and can be         subject to a wide variety of cooling condition to give bainitic         structure namely vermiculite cooling, air cooling, oil cooling,         polymer quenching or water quenching exhibit bainitic structure         and a wide variety of properties in the ultra high strength         range with good ductility and reasonable toughness.     -   4. The steel could be manufactured in large tonnages and present         study has resulted in a 35 MT electric arc furnace steel making         furnace followed by continuous casting and it could be hot         forged and hot rolled without defects at a temperature range         between 850 and 1280° C. The steel developed was extensively         studied for mechanical properties in as-rolled, as-forged and         closed die forging condition followed by air cooling where         excellent Ultra high strength-ductility properties could be         achieved.     -   5. The components like front axle beam have been analysed for         fatigue performance. The steel shows at least five times the         fatigue life of the corresponding alloy steel component. Thus,         the steel developed has a potential opportunity to replace         expensive alloy steels processed by quenching and tempering         treatment and produce lightweight component with better strength         to weight ratio. Weight reduction of structural component of at         least 10% can be achieved.

It is evident from the foregoing disclosure that the invention has the following embodiments:

-   1. An ultra-high strength steel of present invention characterised     in that the said steel comprises in weight percentage carbon in the     proportion of 0.1 to 0.25% manganese in the proportion of 1.2 to     2.5%, silicon in the proportion of 0.5 to 1.7%, chromium in the     proportion of 0.8 to 1.4%, molybdenum in the proportion of 0.05 to     0.1, niobium in the proportion of 0.05 to 0.10 titanium in the     proportion of 0.01 to 0.03%, and the residual elements less than     0.4%, nickel in the proportion of less than 0.4%, vanadium in the     proportion of less than 0.1%. Sulphur in the proportion of 0.03%,     phosphorus in the proportion of less than 0.02%, boron less than 30     ppm, and oxygen in an amount of less than 15 ppm. -   2. A process of making an ultra-high strength steel, the said     process comprising the steps of:     -   melting the steel of the chemical composition disclosed in claim         1, followed by secondary refining and vacuum degassing,     -   casting of said steel through ingot casting or continuous         casting into a bloom,     -   thermo-mechanical processing said cast steel by hot charging the         as-cast ingot or the continuously cast bloom, or alternatively,         cold-charging them in a furnace for further deformation,     -   in the case of hot deformation, initially soaking said ingot or         said bloom at a temperature range between 1280° C. and 1220° C.,     -   subjecting said soaked bloom to hot forging or hot rolling,     -   carrying out reduce rolling,     -   hot finishing the material at a temperature range 1000 to 900°         C.,     -   control cooling the hot deformed steel in any of the cooling         medium selected from a group comprising vermiculite, air, oil,         polymer, or water. -   3. A process of making forged components with an ultra-high strength     steel of embodiments 1 and 2, comprising the steps of:     -   heating and soaking of rolled bar made using the process         disclosed in claim 2 at a temperature range between 1280° C. and         1220° C.     -   hot forging of component.     -   finish forging the component at a temperature range 1000 to 900°         C.,     -   cooling the finish forged component in any of the cooling medium         selected from a group comprising controlled cooled, air, oil, or         polymer. -   4. A process of making an ultra-high strength steel as per     embodiment 1 and 2, characterised in that melting in the step of     casting is carried out using an induction furnace or an electric arc     furnace or a basic oxygen furnace. -   5. A process of making an ultra-high strength steel, as per     embodiment 4, characterised in that the step of thermo-mechanical     processing is carried out by a technique selected from a group     comprising hot rolling, or closed die forging, or extrusion. -   6. A process of making forged components as disclosed in embodiment     3, wherein the step of hot forging of component comprises the steps     of preform manufacturing for blocker forging and finisher forging,     followed by cooling. -   7. A process of making forged components as disclosed in embodiment     6 wherein the step of preform manufacturing comprises the steps of     reduce rolling and bending.

While the above description contains much specificity, these should not be construed as limitation in the scope of the invention, but rather as an exemplification of the preferred embodiments thereof. It must be realized that modifications and variations are possible based on the disclosure given above without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents. 

1. An ultra-high strength steel of present invention characterised in that the said steel comprises in weight percentage carbon in the proportion of 0.1 to 0.25% manganese in the proportion of 1.2 to 2.5%, silicon in the proportion of 0.5 to 1.7%, chromium in the proportion of 0.8 to 1.4%, molybdenum in the proportion of 0.05 to 0.1, niobium in the proportion of 0.05 to 0.10 titanium in the proportion of 0.01 to 0.03%, and the residual elements less than 0.4%, nickel in the proportion of less than 0.4%, vanadium in the proportion of less than 0.1%. Sulphur in the proportion of 0.03%, phosphorus in the proportion of less than 0.02%, boron less than 30 ppm, and oxygen in an amount of less than 15 ppm.
 2. A process of making an ultra-high strength steel, the said process comprising the steps of: melting the steel of the chemical composition disclosed in claim 1, followed by secondary refining and vacuum degassing, casting of said steel through ingot casting or continuous casting into a bloom, wherein said secondary refining comprises the step of killing with aluminium to bring the oxygen level in said steel to under 15 ppm, thermo-mechanical processing said cast steel by hot charging the as-cast ingot or the continuously cast bloom, or alternatively, cold-charging them in a furnace for further deformation, in the case of hot deformation, initially soaking said ingot or said bloom at a temperature range between 1280° C. and 1220° C., subjecting said soaked bloom to hot forging or hot rolling, carrying out reduce rolling, hot finishing the material at a temperature range 1000 to 900° C., control cooling the hot deformed steel in any of the cooling medium selected from a group comprising vermiculite, air, oil, polymer, or water.
 3. A process of making forged components with an ultra-high strength steel, characterised in that said process comprises the steps of: heating and soaking of rolled bar made using the process disclosed in claim 2 at a temperature range between 1280° C. and 1220° C., hot forging of component, finish forging the component at a temperature range 1000 to 900° C., cooling the finish forged component in any of the cooling medium selected from a group comprising controlled cooled, air, oil, or polymer.
 4. A process of making an ultra-high strength steel as claimed in claim 3, characterised in that melting in the step of casting is carried out using an induction furnace or an electric arc furnace or a basic oxygen furnace.
 5. A process of making an ultra-high strength steel as claimed claim 4, characterised in that the step of thermo-mechanical processing is carried out by a technique selected from a group comprising hot rolling, or closed die forging, or extrusion.
 6. A process of making forged components as claimed in claim 3, characterised in that the step of hot forging of component comprises the step of preform manufacturing for blocker forging and finisher forging, followed by cooling.
 7. A process of making forged components as claimed in claim 6, characterised in that the step of preform manufacturing comprises the steps of reduce rolling and bending. 